Apparatus for drying finely divided particles



Nov. 28, 1967- P. J. BARR 3,354,555

APPARATUS FOR DRYING FINELY DIVIDED PARTICLES Filed Jan. 5, 1966 g QKWATTORNE)? I United States Patent Ofifice 3,354,555 Patented Nov. 28, 1957 3,354,555 APPARATUS FOR DRYING FINELY DIVIDED PARTICLES Peter Joachim Barr, 2 Dorset Square, London, England Filed Jan. 3, 1966, Ser. No. 518,461 6 Claims. (Cl. 34-57) ABSTRACT OF THE DISCLOSURE Apparatus for flash drying finely divided material whilst conveyed in a stream of hot gas comprising means for mixing wet finely divided particles with relatively dry finely divided particles and for disintegrating the aggregates so formed in the presence of a current of warm gas, a main drying duct, means for passing a stream of gas through said main drying duct, means for heating the gas entering said duct, means for introducing particles resulting from the distintegration of said aggregates into said main drying duct at a point downstream from said heating means, a separator means arranged on the outside wall of a bend of the main drying duct comprising an opening with an adjustable deflector, duct means for recycling a stream of particle-laden gas from said opening back to the mixer and distintegrator means and means for separating product particles proximate the outlet end of the main drying duct.

The present invention relates to a process and apparatus for drying finely divided particles whilst suspended in a stream of hot air or gas. It is well-known to flash dry finely divided particles in this way. Where the particles are of such a nature that they may be burnt or otherwise damaged if their temperature rises too high, certain problems arise in the flash drying process. In order to achieve economy in plant requirement it is conventional to utilise air at temperature much in excess of that at which the particles themselves might be burnt. It is thus therefore necessary to ensure that the particles have a high specific surface at the moment when they are exposed to the highest temperature Zone of the hot air stream, so that their own surface temperature does not rise excessively, but remains comparatively cool owing to the rapid conversion of their liquid content into vapour at their surface.

It is well-known in the production of animal feeding stuffs from the solid residues, which arise in the prodnction of corn starch, that surface burning of the particles is liable to occur.

For convenience, the apparatus of the present invention will hereinafter be described with reference to the processing of such solid residues, but it will be readily understood that the apparatus finds equal applicability where other heat-sensitive particles are to be dried.

In the processing of the finely ground residues from the production of corn starch, the material to be processed may have an initial moisture content as high as 60%. Because of this high moisture content the particles are far too cohesive to permit them to be dried by flash drying in a hot air stream and it is therefore conventional to mix these wet particles with particles which have already been fully dried. In conventional practice the incoming wet particles and the fully dried particles are thoroughly mixed together. This mixing results in aggregates, which are sufliciently non-cohesive to permit them to be transported in a pneumatic duct. In conventional practice these aggregates of dry and wet particles are fed into a disintegrator, through which the whole hot air input of a flash drying system is blown, so that the coarse aggregates are subjected to the hot air having the highest temperature in the flash drying system. Since at this stage the ratio ofv surface to mass of the coarse aggregates is somewhat low, there is a tendency for the aggregates to become overheated and damaged, although much moisture is flashed off from the aggregates whilst they are in the disintegrator.

It is an object of the present invention to provide a process and apparatus for flash drying moist particles in such a way that the damage caused during the disintegration treatment in a conventional system is obviated. Basically this object is attained by ensuring that the temperature of the air blown through the disintegrator during the disintegration of the aggregates produced by the mixing of wet and dry particles is kept at a value well below the maximum temperature in the system, so that during their disintegration the aggregates are not exposed to the same high temperature as the individual particles, formed by disintegration of the aggregates, are exposed after they leave the disintegrator. At the same time it is desirable to be able to maintain as high a temperature of the air within the disintegrator as is consistent with the avoidance of surface damage of the particles in the aggregates which are being disintegrated and simultaneously dried.

After disintegration of these aggregates the fine particles produced by disintegration are carried forward by the air stream into the main drying duct, where the air temperature conditions are much higher. In order to en? sure the drawing out of the disintegrated particles from the disintegrator, the stream of air leaving the disintegrator preferably enters the main drying duct through an injector, so that the main mass of air entering the duct from a heater is effective to draw the disintegrated particles out from the casing of the disintegrator. Since it is probable that the temperature of the air recycled into the disintegrator may be at a temperature too low to cause flash drying in the disintegrator at a sufliciently rapid rate, it is preferred to arrange that there shall he means for leading a part of the main hot air stream into the body of the disintegrator, so as to permit the air temperature within the body of the disintegrator to be raised to an acceptably high level.

It is already well-known in the flash drying of airsuspended particles to separate drier particles from wetter particles by dividing the air stream in which the particles are carried, as the air stream passes around a sharp bend. In accordance with well-known principles the heavier wetter particles will be carried in the portion of the air stream which lies towards the outside of the bend. In the apparatus of the present invention this principle is used to remove a fraction of the partially dried particles for further drying and for admixture with wet particles entering the drying system. The air in which the particles, shaved off from the drying system, are suspended, is carried back to the disintegrator. It is also preferred to introduce additional air from atmosphere into the air stream shaved ofi from the main drying duct. Theadditional air drawn in from atmosphere may be heated, so that this provides an additional control over the temperature of the air which is drawn into the casing of the disintegrator.

In a preferred arrangement of the present invention the stream of heavier, moister particles which are shaved 0% from the main drying duct, is divided into two parts. One portion of this stream is dropped into the mixer and mixed with fresh undried material, the other is carried into the disintegrator with the stream of air shaved off from the main drying duct and the additional air drawn into the mixer, the mixer itself becomes overloaded and in turn overloads the disintegrator with an excess of heavy aggregates. By-passing some of the recycle stream direct into the disintegrator relieves both mixer and disintegrator. The recycle stream that reaches the disintegrator direct does so in air-borne condition. Whilst having a beneficial sandblasting effect superimposed upon the impact of the disintegrator beater upon the heavy aggregates, this stream already enters the disintegrator in airborne condition and therefore requires only a fraction of the power that would be consumed for handling the same amount of solids enclosed in the heavy aggregates coming from the mixer.

Although it is within the invention to utilise a combincd mixer and disintegrator for mixing together the wet feed with the whole of the partially dried particles in the presence of the air shaved off from the main drying duct, such arrangement is not preferred because it is much more difficult to control than a system in which one part of the recycled partially dried particles is mixed in a mixer with the wet feed, whilst another part of the partially dried particles is carried with the air stream'into the casing of the disintegrator.

In order that the invention may be more fully comprehended reference is hereinafter made to the accompanying drawing, which shows in diagrammatic form an apparatus made in accordance with the invention.

In the apparatus shown the major part of the air passing through the main drying duct 1 is drawn in through a heater 2, in which it may be heated to a high temperature, for example, l,000 F. A stream of particles enters the main drying duct 1 through an injector 3, whilst carried in an air stream which has been led through the casing of the conventional disintegrator 4, in which relatively coarse aggregates are broken down into fine particles by the impact of the rapidly rotating beater of the disintegrator. The stream of air in the main drying duct 1 is induced by a powerful blower 5 which draws air out axially of a cyclone separator 6. In addition to the blower 5 it is also possible to utilise a blower at a position adjacent the heater 2. However, in practice it is preferred to maintain a slight sub-atmospheric pressure in the duct 1. In accordance with the well-known principles of socalled ring-drying systems, at a point on the sharp bend in the drying duct 1 immediately preceding the entry into the cyclone separator 6, there is a movable vane 7, which permits a variable proportion of the air stream and the particles entrained therein to be diverted into a recycling duct 8. Also provided at the same point is an atmospheric air intake 9 and an associated heater 10. The proportion of air drawn into the recycling duct 8 is governed by the position of a pair of movable vanes 11 and 12, which define an injector opening. The vane 11 also cooperates with the movable vane 7 to define the size of the opening through which particle-laden air may be drawn off from duct 1. The recycling duct 8 divides into a chute 14, in which is maintained a plug or column of particles and an air-recycle duct 15, which leads into the casing of the disintegrator 4. A further controllable vane 16 is provided at the junction of the ducts 14 and 15 for the purpose of controlling the proportion of the particles carried in the air stream passing through the duct 8 respectively entering the column 14 and the duct 15. The particles in the chute 14 are continuously supplied by a feed worm 17 to a mixer 18 in which they are mixed with wet feed, which enters through a chute 19. It is also possible to supply to the mixer 18 a proportion of the fully dried particles which are drawn otf by a feed worm 20 from the bottom of the cyclone 6. It will be appreciated that the particles drawn oil from the bottom of cyclone 6 constitute the product.

It will be observed that in addition to air recycled through duct 15, the air stream passing through the casing of the disintegrator 4 may be supplied with hot air led directly from the heater 2 through an air supply passage 22, which is preferably provided with a control to permit the temperature of the air stream passing through the casing of the disintegrator 4 to be controlled to a desired value.

In a typical system the temperature of the air rising in the drying duct 1 falls quite rapidly as moisture is flashed from the finely divided particles carried in it. As already indicated, the air entering the duct from the heater 2 may be at a temperature of about 1,000 P. The temperature of the air stream may typically be aproximately 500 F. near the top of the vertical leg of the drying duct and will have been further reduced to about 200 F. at the entry to the cyclone separator. Typically the average moisture content of the particles entering the cyclone separator in the stream of air will have been reduced to about 10%, whilst the moisture content of the heavier wetter particles entering the duct 8 may average about 20%. The average moisture content of the particles entering the duct 8 will depend upon the setting of the vane 7 and the average moisture content will naturally be lower, the larger the portion of the main air stream being shaved otl by means of the vane 7.

The temperature of the air stream in the duct 8 is preferably raised to about 300 F. by the heater 10 at the inlet end of the atmospheric air intake 9. This ensures that the temperature of the air in the recycle duct 15 is maintained at a value of about 250-300 F. Such a temperature is, however, insufficient to ensure rapid flash drying of moisture from the aggregates entering the disintegrator 4, which are fed from the mixer 18 by the feed worm 23. The temperature of the air stream entering the disintegrator is preferably about 750 F. and this value may readily be achieved by operating the controls governing the rate of supply of air through the duct 22. The setting of this control will naturally depend upon the setting of the vanes 7, 11 and 12. Owing to the flash drying of the aggregates which occur during the disintegration process, the temperature of the air stream leaving the disintegrator for emission through the injector 3 is in the region of BOO-400 F.

It will readily be seen that by this arrangement the temperature of the air stream in the disintegrator is maintained at a sufliciently low value to avoid surface-burning of the material. The air temperature in the disintegrator is readily controllable through a wide range of values quite independently of the temperature of the air entering the main drying duct 1. As already stated above, the operator will arrange that the temperature of the air entering the disintegrator will be maintained at as high a value as is consistent with the avoidance of surface burning or surface damage of the aggregated particles passing through the disintegrator.

In one modification of the above-described apparatus the mixer 18 and disintegrator 4 may be combined into a known rnixer-disintegrator. In such event, the column tube 14 and vane 16 are dispensed with. However, this modification is not preferred over the apparatus illustrated in the accompanying drawing.

In another modification of the apparatus illustrated in the drawing the injector 3 may be dispensed with where the disintegrator 4 is of the kic mill type, imparting sufficient energy to the particles to 'sure that they pass into the more rapidly moving stream of air in the main drying duct 1.

Other simple alternatives will be obvious to those skilled in the art.

I claim:

1. Apparatus for flash drying finely divided material whilst conveyed in a stream of hot gas comprising means for mixing wet finely divided particles with relatively dry finely divided particles and for disintegrating the aggregates so formed in the presence of a current of warm gas, a main drying duct, means for passing a stream of gas through said main drying duct, means for heating the gas entering said duct, means for introducing particles resulting from the disintegration of said aggregates into said main drying duct at a point downstream from said heating means, a separator means arranged on the outside wall of a bend of the main drying duct comprising an opening with an adjustable deflector, duct means for recycling a stream of particle-laden gas from said opening back to the mixer and disintegrator means and means for separating product particles proximate the outlet end of the main drying duct.

2. Apparatus according to claim 1, further including an auxiliary atmospheric air inlet duct adjacent said separator means for introducing atmospheric air into said recycling duct and variable injector means in said duct for controlling the rate of entry of gas therethrough.

3. Apparatus according to claim 2, further including 15 heating means in said auxiliary atmospheric air inlet duct.

4. Apparatus according to claim 1, further including means for introducing hot gas from the inlet end of the main drying duct to the means for disintegrating the aggregates.

5. Apparatus according to claim 1, comprising means in said recycling duct for diverting a variable proportion of the particles passing therethrough to a collecting space, mixer means, means for introducing fresh particulate material of high moisture content into said mixer means and means for drawing recycled partially dried particles from said collecting space and introducing them into said mixer means, means for feeding mixed material from said mixer means to said disintegrator, means for introducing a stream of particle-laden air into said disintegrator from said recycling duct and means for introducing heated air from a point adjacent the heater in the main drying duct for admixture with said stream of particle-laden air.

6. Apparatus according to claim 1, comprising means in said recycling duct for diverting variable proportions of particles into said mixer and said disintegrator, further including means for introducing hot gas from the inlet end of the main drying duct into said disintegrator.

References Cited UNITED STATES PATENTS 1,627,766 5/1927 Bergman 34-57 2,156,924 5/ 1939 Schneider 3457 20 2,363,282 11/1944 Arnold 34-57 2,313,956 3/1943 McGrane 34-57 FREDERICK L. MATTESON, IR., Primary Examiner. A. D. HERRMANN, Assistant Examiner. 

1. APPARATUS FOR FLASH DRYING FINELY DIVIDED MATERIAL WHILST CONVEYED IN A STREAM OF HOT GAS COMPRISING MEANS FOR MIXING WET FINELY DIVIDED PARTICLES WITH RELATIVELY DRY FINELY DIVIDED PARTICLES AND FOR DISINTEGRATING THE AGGREGATES SO FORMED IN THE PRESENCE OF A CURRENT OF WARM GAS, A MAIN DRYING DUCT, MEANS FOR PASSING A STREAM OF GAS THROUGH SAID MAIN DRYING DUCT, MEANS FOR HEATING THE GAS ENTERING SAID DUCT, MEANS FOR INTRODUCING PARTICLES RESULTING FROM THE DISINTEGRATION OF SAID AGGREGATES INTO SAID MAIN DRYING DUCT AT A POINT DOWNSTREAM FROM SAID HEATING MEANS, A SEPARATOR MEANS ARRANGED ON THE OUTSIDE WALL OF A BEND OF THE MAIN DRYING DUCT COMPRISING AN OPENING WITH AN ADJUSTABLE DEFLECTOR, DUCT MEANS FOR RECYCLING A STREAM OF PARTICLE-LADEN GAS FROM SAID OPENING BACK TO THE MIXER AND DISINTEGRATOR MEANS AND MEANS FOR SEPARATING PRODUCT PARTICLES PROXIMATE THE OUTLET END OF THE MAIN DRYING DUCT. 